1. Field of the Invention
The present invention relates to a protection circuit for a wind turbine generator and more particularly to a PWM Brake Control Circuit with novel speed sensing.
2. Description of the Prior Art
Wind turbine generator systems are generally known in the art. Examples of such systems are disclosed in U.S. Pat. Nos. 4,565,929; 5,506,453; 5,907,192; 6,265,785; and 6,541,877. Such wind turbine generator systems are also described in U.S. Patent Application Publication Nos. US2002/0117861; 2005/0034937; 2005/0230979; 2005/0236839; 2006/0006658; and 2006/0012182. Such wind turbine generator systems are known to include pole mounted turbine generators. The wind turbine generator systems also include an inverter and protection circuitry.
Due to the ever-increasing demand and increasing cost for electrical power, renewable energy sources, such as wind turbine generator systems, are becoming more and more popular for generating electrical power. Such wind turbine generator systems are known to be used individually to generate supplemental or excess power for individual, residential or light industrial users to generate electrical power in the range of 1-2 kw. Such wind turbine generator systems are also known to be aggregated together, forming a wind turbine generator farm, to produce aggregate amounts of electrical power. It is also known that unconsumed electrical power generated by wind turbine generators is connected to the utility power grid.
Such wind turbine generators are known to include a wind turbine, which includes a plurality of turbine blades connected to a rotatable shaft. The rotatable shaft is rigidly connected to a direct current (DC) generator. Wind causes rotation of the wind turbine which acts as the prime mover for a DC generator. The generator, for example, a self-excited generator, generates DC electrical power.
One problem with such systems is that wind speeds are not constant. As is known in the art, the voltage output of the generator is a cubic function of the speed of the speed of the wind. As such, the effect of wind gusts on the wind turbine generator must be controlled to prevent damage to the wind turbine generator.
Some wind turbine generator systems are known to use some type of mechanical braking to protect the wind turbine generator from an over speed condition. For example, U.S. Pat. No. 5,506,453 utilizes the pitch of the wind turbine blades to protect the wind turbine from over speed. In particular, the blades of the wind turbine are mechanically coupled to a rotatable mechanical hub. The blades are configured so as to be rotatable about their longitudinal axis relative to the hub allowing the pitch of the turbine blades to be varied. The pitch of the blades is turned in such a way as to create braking of the wind turbine.
Other known systems utilize mechanical brakes, such as disclosed in U.S. Patent Application Publication No. US 2005/0034937. Yet other systems disclose the use of aerodynamic-type brakes as well as mechanical brakes, for example, as disclosed in U.S. Pat. No. 6,265,785, to protect the wind turbine from over speed.
While mechanical brakes do an adequate job of protecting the wind turbine generator from over speed, mechanical braking systems do little to optimize the operational time and thus power output of the wind turbine generator. Moreover, such mechanical braking systems are mechanically complex and are, thus, relatively expensive.
As such, electrical braking systems have been developed to control over speed of wind turbine generator systems. For example, Japanese Patent Publication JP2000-179446 discloses an electrical braking system known as a Brake Relay for a wind turbine generator. The system disclosed in the Japanese patent publication includes a Brake Relay whose contacts are connected across the output terminals of the generator. In this application, since the speed of the turbine is proportional to the voltage at the generator output, the generator voltage is used to trigger this system. When an over speed condition (i.e. over voltage) is detected, the Brake Relay is energized which, in turn, shorts out the output terminals of the generator, which loads the generator and causes it to slow down and stop.
Such Brake Relays are known to be under the control of a Brake Relay Control Circuit, which actuates the Brake Relay as a function of a voltage representative of the generator output voltage. As mentioned above, output voltage of the generator is a cubic function of the wind. Thus, wind gusts are known to cause the generator voltage to rise high enough to trip the Brake Relay, thus causing a disruption of power delivered to the AC power grid.
Referring to FIG. 1, a conventional wind turbine generator system 20 is illustrated, generally identified with the reference numeral 20. The wind generator system 20 includes a generator 22, such as, a self-excited DC generator, a wind turbine (not shown) and an inverter 28 and generator protection circuitry. The wind turbine functions as a prime-mover for the generator 22. The generator 22 generates a DC voltage across its output terminals 24, 26 as a cubic function of the wind. In as much as the generator 22 is directly coupled to the wind turbine, the rotational speed of the turbine and generator is directly proportional to the wind speed. As such, the output voltage at the generator terminals 24 and 26 is a cubic function of the wind speed.
The output terminals 24, 26 of the generator 22 are coupled to an inverter, shown within the block 28. The inverter 28 converts the DC output voltage, available at the output terminals 24, 26 of the generator 22, to an AC voltage suitable for connection to a utility AC power grid, generally identified with the reference numeral 30. The AC power grid 30 may be a phase to phase 230/240 Volts AC, suitable for residential, commercial and industrial application. In the exemplary embodiment shown, shown, the inverter 28 generates a phase to phase voltage across two output phases L1 and L2, for example, 230/240 Volts AC.
Depending on the configuration of the utility AC power grid 30, the inverter 28 may also include a ground conductor for use with utility AC power grids which are 230/240 Volts AC with a center tap ground, for providing 230/240 Volts AC phase to phase and 115/120 Volts AC phase to ground. In such a system, the inverter ground conductor (not shown) would be electrically coupled to the utility center tap ground. The wind turbine generator systems 20 may be configured to be connected to various configurations of the utility AC power grid 30.
The phase to phase output L1 and L2 Of the inverter 28 is connected to the utility AC power grid 30 by way of an AC grid relay 32. As mentioned above, the grid relay 32 is used, among other things, to disconnect the wind turbine generator system from the utility AC power grid 30 during abnormal conditions relating to the voltage, phase or frequency of the AC power grid 30. The grid relay 32 is under the control of an AC Relay Control Circuit 34. The AC Relay Control Circuit 34 monitors the phase of the output of the inverter 28 and the phase of the utility AC power grid 30. When the phase of the inverter output is synchronized with the phase of the utility AC power grid 30, the AC Relay Control Circuit 34 causes the grid relay 32 to connect the two together.
In order to protect the wind turbine generator system 20 from damage from over speed resulting from wind gusts, as mentioned above, some wind turbine generator systems 20 include a Brake Relay 36. The Brake Relay 36 is connected across the output terminals 24, 26 of the generator 22. The Brake Relay 36 may be an electromechanical relay that shorts the output terminals 24, 26 of the generator 22 together for a nominal time period during an over speed condition. Shorting the terminals 24, 26 of the generator 22 together creates a load on the generator 22 and slows down and eventually stops the generator 22, thus acting as an electronic brake. In order to protect the generator from damage due to the variability of the wind speed, many known wind turbine generator systems 20, continuously monitor the output voltage of the generator 22 at a DC Measurement Point. When the output voltage of the generator 22 exceeds a predetermined threshold voltage, for example, 320 Volts DC (“first threshold”), indicative of an over speed condition, the conventional Brake Relay Control Circuit 38 activates the conventional Brake Relay 36, which shorts the terminals 24, 26 of the generator 22 and maintains the short circuit condition, thus shutting down the generator 22, for a nominal time period, such as 10 seconds or more, for example. This shut down condition causes the wind turbine generator system 20 to be off-line during a wind condition in which the system could be delivering maximum power to the utility AC power grid 30. As such, this condition makes wind turbine energy systems 20 less desirable as a renewable energy source.
In order to provide a protection system which minimizes the disruptions in power while still providing sufficient protection to the turbine-generator system, it is necessary to trigger a generator protection circuit based upon the over speed of the generator However, wind turbine generators are pole mounted. Thus any speed sensors to measure the speed of the turbine generator need to be mounted on the turbine-generator and cabled from the pole top mounted generator. Considering the number of wind turbine generators in a wind turbine generator farm, such cabling would substantially add to the cost of the overall wind turbine generator system. Thus, there is a need for protection circuitry for a wind turbine generator system that is triggered on over speed of the generator which does not require a sensor mounted on the turbine generator mounted on a pole top.